1. Technical Field
The invention broadly relates to data processing methods and apparatus and more particularly relates to improvements in I/O control between a communications network and the data processor.
2. Background Art
Data communications networks which interconnect data processing nodes, are not only inherently complex, but have evolved into a variety of forms. Information flowing between I/O terminal devices and application programs residing on a remote host computer, require the coordination among the terminal design characteristics, the application requirements, the operating system in the main computer systems, and the characteristics of the communications link. General purpose communications controllers have been developed, containing a stored program front end that can attach to the I/O channel of a host data processor, to provide teleprocessing control functions. Another class of control functions, implemented with software and known as telecommunications access methods, are fundamentally of two types, direct control and queued. For example, the Basic Telecommunications Access Method (BTAM) provides for direct control over telecommunications operations while appropriately interfacing with the operating system of the host computer. The Virtual Telecommunications Access Method (VTAM) was later developed to match the technology advances of virtual memory machines and Synchronous Data Link Control (SDLC), along with having a compatibility with new programmable communications controllers and providing a facility for distributed processing. VTAM is described in the IBM publication "Virtual Telecommunications Access Method (VTAM)--Concepts and Planning," GC27-6998-3, June 1976. Additional description can be found in the article by H. R. Albrecht, et al., "The Virtual Telecommunications Access Method--A Systems Network Architecture Perspective," IBM Systems Journal, Vol. 15, No. 1, pp. 53-80, 1976. An overall telecommunications architecture plan known as Systems Network Architecture (SNA) significantly advanced the state of the art in teleprocessing software systems by providing a unified design for the functions and structure of data communications products. SNA and SDLC are described in detail in the book by R. J. Cypser, "Communications Architecture for Distributed Systems," Addison Wesley Publishing Co., Reading, Mass., 1978. SNA is also described in the publication "SNA Technical Overview," order No. GC30-3073, published by the IBM Corporation and available through IBM branch offices. A second major communications network architecture has evolved, known as the Defense Data Network (DDN) which consists of a packet-switched network providing data communications services between remote data processing hosts and terminals. The Defense Data Network (DDN) is described in the NTIS publication AD-A166324 entitled "DDN (Defense Data Network) Protocol Handbook," Vol. 1, DoD Military Standard Protocols, December 1985 by E. J. Feinler, et al., and its companion volumes 2 and 3. Additional information can be found in the NTIS publication AD-A137427 "Defense Data Network X.25 Host Interface Specification," December 1983. These two major data communications architectures have incompatible protocols which prevents communications between machines designed for either one or the other type of architecture. Prior art I/O control units attached to an I/O channel of a host computer using the SNA-type protocol, cannot communicate with the non-SNA-type protocols and methodologies. For example, a terminal user on an IBM 3270 terminal conventionally attaches to host applications over an IBM 3274 control unit, but IBM 3274 control units are not compatible with Defense Data Network communication protocols and cannot operate using them. What is desired is a programmable I/O controller which can concurrently handle a plurality of communications protocols between a host computer and two or more diverse and mutually incompatible data communications architectures.